This disclosure relates generally to fuel cells and methods of their manufacture and use. More particularly, the present disclosure relates to fuel cells capable of operation by electrolyzing compounds in a biological system and methods of their manufacture and use.
There has long been interest in techniques for providing electrical power from a power source that utilizes biological matter freely available in the environment. One such area of interest is in developing microbial fuel cells (MFCs) as a means to treat wastewater more efficiently by breaking down organic waste products and converting the energy of their chemical bonds into electricity and hydrogen. According to the May 2004 issue of Environmental Science & Technology, 46 trillion liters of household wastewater are treated annually in the United States at a cost of $25 billion. Importantly, the electricity required—mostly for aeration—constitutes 1.5% of the electricity used in the nation. Other nations have similar statistics.
Recently, researchers have shown the feasibility of using microbial fuel cells to generate electricity wherein the source of electricity is the chemical energy contained in the bonds of organic compounds which are a principle constituent of wastewater. Using laboratory scale microbial fuel cell reactors comprising a special anode, a simple cathode and a suitable proton exchange membrane (PEM) to separate the wet anode and cathode portions of the microbial fuel cell, energy densities in the order of 30 watts/cubic meter have been generated.
The process uses bacteria, living in biofilms on the special anode, to break down the organics, separating electrons from protons. These electrons and protons then travel to the cathode, the former via an external wire, the latter by diffusing through the electrolyte which is generally a substance that does not conduct electricity readily. In the electricity-generating microbial fuel cells, the protons and electrons combine at the cathode with oxygen to form water. This consumption of the electrons allows more electrons to keep flowing from the anode to the cathode as long as there is a source of chemical bonds (i.e. organic waste) to fuel the reaction.
The first microbial fuel cells produced between 1 and 40 milliwatts of power per square meter (mW/m2) of anode electrode surface area. In the past year researchers have been able to increase this more than 10 fold by demonstrating that they could generate power in the range of up to 500 mW/m2 using domestic wastewater and 1,500 mW/m2 with a surrogate for waste water comprising glucose and air. Demonstration of these latter power densities has encouraged much discussion about the technical requirements to enable profitable commercial power production. In brief, improvements to the output power density by another factor of at least 10 will be required in order to make the technology attractive on a commercial scale.
Today, scale-up for commercial uses has several challenges. For example, the current laboratory-scale prototypes use materials that aren't sturdy or robust enough to be used in a commercial system. Further, experimental microbial fuel cells are presently small in size and would need to be much bigger (to compensate for the low power density), undoubtedly and unfortunately this would greatly increase the distance between anode and cathode which would slow diffusion of hydrogen from the former to the latter, further damping efficiency. To be competitive, the power density must more than double the maximum achieved so far i.e. 8,500 mW/m2.